Docs should explain IslamicObservational

[hsivonen]

Currently https://unicode-org.github.io/icu4x/rustdoc/icu_calendar/cal/struct.IslamicObservational.html says is "Islamic Observational Calendar (Default)".

The docs should explain at least:

• That "observational" means an astronomical simulation of moon observability from a reference location on Earth.
• That the reference location is Cairo and why it is Cairo. (The code comment points to Reingold's code, which says Cairo is a "sample location".)
• Why this is "(Default)".
• What use cases this variant is for relative to the other three variants.

[sffc]

Would it be smart to name the calendar IslamicCairo instead of IslamicObservational? Then we could add other reference locations on an as-needed basis.

[sffc]

Based on the regional calendar preferences, it seems that islamic is the preferred Hijri caliendar in EG and IL. In that sense, choosing Cairo as a reference point is maybe not a bad choice, but it would be good to gather data on the expectations of users in IL.

[hsivonen]

Would it be smart to name the calendar IslamicCairo instead of IslamicObservational? Then we could add other reference locations on an as-needed basis.

Is Cairo already included on an as-needed basis as opposed to Reingold using it as a sample?

Based on the regional calendar preferences, it seems that islamic is the preferred Hijri caliendar in EG and IL. In that sense, choosing Cairo as a reference point is maybe not a bad choice, but it would be good to gather data on the expectations of users in IL.

Many more regions have islamic their preferred Hijri calendar variant the CLDR data (though no region has it as the most preferred calendar in the data), but what does that data mean?

Does it mean that CLDR attests the regions to prefer the specific parameters used in ICU4C/J?

Does it mean that the regions prefer observational in the sense of actual local-to-region observation (as opposed to the parameters either in ICU4X or ICU4C)?

Does it mean something else?